For some time, I have been thinking of writing a fictional work titled Albert's Regrets. When you think of the life of Albert Einstein, you would think that his greatest regret scientifically has been the fact that he could not complete his unified theory of physics. He might regret his treatment of his first wife, Mileva, both on personal and professional terms. Some estimate that Mileva contributed to theory of relativity much more than is usually thought.
Albert's greatest regret, however, would have been the signing of the letter to President Roosevelt to develop the atomic bomb. There was historical urgency to do so, to be sure. On the other hand, when you know Einstein's deeply pacifist views, he would have been the last person you would expect to be involved in such a conduct. So there is a genuinely profound food for thought in the circumstances in which Albert Einstein wrote that crucial letter.